1. Field of the Invention
The present invention relates in general to composite foam products and processes for making the composite foam products.
2. Description of Related Art
Foams range in consistency from rigid materials suitable for structural use to flexible substances for soft cushions and packaging materials. These foams range in cellular formation from open or interconnecting-cell foams to closed or unicell foams. The cell structure may range from large to fine. Electrical, thermal, mechanical, and chemical properties can be varied within wide limits depending on the thermoplastic resin composition and the method chosen to create the foam.
Foam products can be produced by a wide variety of processes. In some methods, a gaseous “blowing agent” is added to the foamable material during processing. Other methods involve producing a gaseous blowing agent in the foamable material during processing. Blowing agents generally work by expanding a thermoplastic resin to produce a cellular structure having far less density than the resin from which the foam is made. Bubbles of gas form around “nucleation sites” and are expanded by heat or reduced pressure or by a process of chemical reaction in which a gas is evolved. Additives may be incorporated into the resin to promote nucleation for a particular blowing agent and, consequently, a more uniform cell distribution.
The gas from the blowing agent used to form the cellular structure of the foam may initially reside in the cells of most closed cell foams at an increased pressure. As the foam is exposed to the environment, however, the gaseous blowing agent permeates out of the foam cell structure while the component gases of the ambient air permeate into the foam cell structure. That is, there is an influx of nitrogen and oxygen from the surrounding air into the foam, and there is also an efflux of the blowing agent from the foam. The rate of the influx/efflux varies on parameters such as the molecular size and polarity of the gaseous blowing agent(s), the type of polymers(s) comprising the foam, the cell size, the cell structure, the pressure and temperature, and the ambient conditions. Nonetheless, the final result is that the pressure inside of the gases inside the foam cells ultimately comes into equilibrium with the ambient pressure. This is true for even closed cell foams since the gas molecules are able to permeate and move through the polymer molecules comprising the foam.